1. Field of the Invention
The present invention relates to a semiconductor apparatus having a strained-Si channel, and more particularly to a semiconductor apparatus having an improved gate insulating film or gate electrode structure.
2. Description of the Related Art
In recent years, as a method of improving electron mobility, which is one of the guidelines of realization of high performance of an Si-MOSFET, a technology which applies a strain to an Si layer has attracted attention. When the strain is applied to the Si layer, its band structure is changed, and scattering of carriers in a channel can be suppressed. Therefore, an improvement in mobility can be expected. Specifically, a compound crystal layer formed of a material having a larger lattice constant than that of Si, e.g., an SiGe compound crystal layer (which will be simply referred to as an SiGe layer hereinafter) having a Ge concentration of 20% is formed on an Si substrate, and an Si layer is formed on this SiGe layer. Then, a strained-Si layer, to which a strain is applied due to a difference in lattice constant, is formed.
It has been reported that a great improvement in the electron mobility which is approximately 1.76 times that of a semiconductor device using a strain-free channel layer can be achieved when such a strained-Si layer is used for a channel of a semiconductor device (J. Welser, J. L. Hoyl, S. Takagi, and J. F. Gibbons, IEDM 94–373).
Further, when realization of a short channel of a MOSFET is advanced in order to improve the electron mobility, the influence of stray capacitance becomes large, and it is difficult to improve the electron mobility as expected. In order to solve this problem, a structure in which the semiconductor channel layer is provided on an SOI (Silicon On Insulator) structure has attracted attention. By introducing this structure, a reduction in stray capacitance or isolation of elements can be facilitated, and realization of a further reduction in power consumption or higher integration can be expected compared with the prior art.
On the other hand, as MOSFETs gradually become minute, in a structure where a gate insulating film/channel layer is formed on a conventional oxide film/Si substrate, it is expected that a limit in realization of minuteness will be reached in 2010 or later years (ITRS Roadmap 2000). Here, as one of the problems which first reach the limit in realization of minuteness as well as a limit in lithography, there is the sudden increase in tunnel leakage current due to a reduction in film thickness beyond several nm, which is against a reduction in power consumption, or the limit in reduction in film thickness of an oxide film that an operation failure due to a deterioration in film quality owing to a reduction in film can be expected.
As a result of an examination concerning an increase in leakage current in an oxide film having a film thickness of 1 nm, a fact that the roadmap of ITRS is adequate has been already demonstrated as actual data (M. Hirose et al., Smicond. Sci. Tecnol. 15,485 (2000)). Therefore, in order to continue realization of minuteness in future and subsequently aim at an improvement in an element characteristic such as offering of sophisticated functions, higher performances, a higher speed and others, an alternative technique of the oxide film as well as an improvement in the channel layer must be developed.
As described above, in a semiconductor device including a strained-Si channel layer in the prior art, although realization of higher performances can be expected as compared with a device in which the channel layer is formed directly on the Si substrate, it is considered that a new technology must be introduced to structures other than the channel layer in the case of aiming at realization of higher performances in future.
Therefore, in a structure using the strained-Si channel layer, there is desired realization of a semi-conductor apparatus which can suppress a deterioration in film quality of a gate insulating film due to realization of minuteness and an increase in leakage current and which can further improve an element characteristic.